Mizuho International Chief Economist Ricccardo Barbieri attributed the fall to lack of detail on how the ECB will assess the euro zone's leading banks, coupled with fears the capital limits insisted on in the tests were more stringent than those currently posed by national regulators.

One hundred and twenty-eight banks will undergo an assessment of their risky assets, the quality of their balance sheets, and the amount of capital they hold. This is designed to test their ability to withstand economic difficulties or "stress".

"Now the ECB has to introduce a harmonized model, which will be potentially very complex. Therefore there could be surprises and deleveraging because of it," Barbieri said, following Draghi's interview with CNBC.

Draghi: Europe needs banking transparency

Mario Draghi, president of the ECB, discusses the "uneven and fragile" European recovery and the need for transparency in the banking sector.

"Over time we will have to acquire more elements to assess what the ECB is really up to… the ECB will carry out checks on liquidity, capital, positions, overall leverage of banks… they are not telling us more for the moment about how they are going to do it, what will be the parameters. That introduces a bit of uncertainty and this is perhaps why today banks sold off," Barbieri added.

James Howat, European economist at independent research firm Capital Economics, agreed that market confidence was hit by a lack of detail issued by the ECB. He added that the stress tests appeared to be tougher than anticipated, particularly with regards to the 8 percent core Tier 1 capital requirement.

"It is difficult gauging the markets' reaction. If they (the ECB) are too severe it hits confidence, but it is also disappointing if they are not strict enough," he told CNBC.